The breath is affected by many factors, such as posture, state of health and frame of mind. The breath, in turn, can be used as a tool to influence these.

There are many practices using breath as the vehicle.
Here I will borrow from the Zen practice of just observing, without attempting to influence the observed, and from the Feldenkrais Method, which aims to reduce unnecessary effort and strain in order to foster a sense of calm and create the right conditions for awareness.

The practice I want to talk about here is the simple act of spontaneous breathing and the not so simple act of observing the breath, just as it is.

You can do a breathing practice while standing, walking, sitting, or lying down. Each one has its challenges. Lying down will reduce the physical challenges for most people.
It can be difficult to find a balanced muscular tone in sitting. “Balanced tone” means that the muscles in the front and the muscles in the back of your trunk (“flexors” and “extensors”) are working equally; neither group more than the other. While there are practices to balance the tone in those muscles, the idea here is not to wait until you have a perfectly aligned spine and evenly balanced flexors and extensors, but to help yourself now, whatever your situation. This, by the way, will have a positive influence on your physical and mental well being, including your muscular tone.

Let’s begin. Minimize those distractions that it is in your power to minimize. Find a firm but comfortable place to lie down. Lie down. Just breathe. Observe. If you are not used to observing the breath these questions might be helpful:

What is rising, my abdomen or my chest?
What is the speed of the in breath?
Is there a pause between the inhalation and the exhalation?
What is the speed of the exhalation?
Is there a pause between the exhalation and the next in breath?

Remember, you are not trying to change anything, but to see what IS.
This simple act frequently brings change -but not always quickly!, so be nice to yourself. If your breath is shallow, or jagged, or in any way irregular or labored there is a reason for that. Be patient with yourself. It frequently takes me longer than I would like to yield to the floor and truly attain relaxation. If you achieve full even breaths and a calm state in a few minutes, excellent; if it takes you longer, it is still excellent that you are practicing.

Even five minutes is good; 40 to 45 is already very therapeutic. Anything you can is acceptable, because that is what you can do,
and whatever you can do is better than no practice.
As the Zen teacher Thich Nhat Hanh said:
“Breathe, you’re alive!”